Many products, especially devices and supplies used in surgical and other medical applications, must be sterilized prior to use. Examples of such products in the medical context include but are not limited to surgical devices, implants, tubing, valves, gauzing, syringes, and protective clothing such as surgical gowns and gloves. Such products and supplies are often packaged prior to being stabilized. One sterilization procedure for such products involves using sterilizing gases that will penetrate pores in the packaging. Steam and ethylene oxide are examples of such sterilizing gases. The gas flows through the pores in the packaging material and sterilizes the instruments contained therein. Over time, the gas will then diffuse out of the package. The packaging serves to protect the instruments during sterilization and to preserve their sterility upon subsequent storage until the packages are opened for use of the product. To allow proper sterilization, packaging for medical products should be sufficiently permeable to sterilization gases to allow the gases to sterilize the product within. To avoid communication after sterilization, the packaging should also prevent the entry of bacteria and pathogens into the package.
Packaging for many medical and sterile supplies includes two components, referred to herein as the base component and the breathable cover. The two components are attached to one another to form such structures as a pouch, which combines two flexible layers, or a rigid container, which uses a rigid base component often in the form of a tub or tray with the breathable cover acting as a lid. The sterile devices are stored between the two layers in a pouch or within the tub, tray, or other space within the rigid base component in a rigid container. The package is completed by sealing the two layers together, often by heating the materials so that a seal is formed using a temperature sensitive adhesive. When the device contained in the package is needed, the package is opened. Such packages are opened commonly and desirably by pulling the two components apart along the seal. Examples of such packages are widely known and include: U.S. Pat. No. 3,991,881 to Augurt, U.S. Pat. No. 4,183,431 to Schmidt et al.; U.S. Pat. No. 5,217,772 to Brown et al.; and U.S. Pat. No. 5,418,022 to Anderson et al.
Seals between components of a package must have sufficient strength to assure that stresses resulting from package handling after assembly will not cause the seal to open before the desired time and will remain impervious to pathogens. Seal strength is commonly expressed as the force required to separate the two sealed layers when holding the layers at facing edges and pulling the layers in opposite directions, commonly referred to as a “T-peel” because peeling results in the two separated portions of the layers forming the arms of the letter “T” with the base of the letter “T” being the portion of the two layers that remain attached until pulled apart. One method used to evaluate seal strength using a “T-peel” is found in American Society of Testing and Materials (ASTM) method F904-98. Other methods for testing seal strength exist, some of which are based on this ASTM method. Many users of such packages specify that the seal have a minimum strength of 0.70 pounds per inch in a T-peel test. Accordingly, seal strength that are at least about 0.70 pounds per inch are especially desirable. In some applications, the seal strength desirably is not so great that one or more of the package components will tear before the seal opens. A desirable seal strength in such applications is thus greater than 0.70 pounds per inch but lower than a value that would result in tearing of one or more of the package components upon opening.
The base component in this type of package should be impervious to bacteria and other pathogens. Typical material used in making base components include, but are not limited to, such polymers as nylon, polyester, polypropylene, polyethylene and polystyrene. Of these materials, nylon, polyester, polyethylene (including but not limited to low density, linear low density, ultra low density and high density polyethylene), and polypropylene are particularly useful for flexible base components. Polyester, polyethylene (including but not limited to high density polyethylene), polypropylene, and polystyrene are examples of polymers that are particularly useful for rigid containers such as tubs or trays. Those skilled in the art will recognize that the preceding lists of base components and materials used in making base components are for illustration purposes only and are not meant to be exclusive.
The breathable cover is typically a nonwoven web, which is a sheet comprised of cellulose fibers, synthetic fibers, or a combination thereof. Different materials, including some fabrics, have been used to form breathable covers for use in medical supply packaging. (As used herein, the term “fabric” is intended to encompass any sheet-like or web material that is formed in whole or in part from a plurality of fibers). One such material comprises webs of polyolefin fibers such as the spunbounded polyolefin material sold under the trademark TYVEK® by E.I. Du Pont De Nemours & Co. Others are webs comprising cellulose fibers or papers that have been saturated with one or more polymers such as acrylates to impart certain qualities to the paper. Such polymer reinforcement improves one or more of dimensional stability, resistance to chemical and environmental degradation, resistance to tearing, embossability, resiliency, conformability, moisture vapor transmission, and abrasion resistance, among others. In addition, saturation of paper-based webs by such emulsions ties down the cellulose fibers so that particulate generation is reduced when the fabric is torn or peeled. Polymer saturated papers provide certain advantages over polyolefin webs. Webs made from polyolefins often lack the suppleness, softness, and drapability that polymer saturated papers may possess. Use of cellulose webs is also a less expensive alternative to the polyolefin webs.
The polymer is normally applied by a saturation process, which involves dipping the formed fabric web into a bath of emulsion or subjecting the fabric web to an emulsion-flooded nip. Alternatively, the webs may be subjected to polymer impregnation while still on the forming wire through the use of various emulsion processes and the like. Polymer impregnation may also occur prior to forming the web as described in International Publication Number WO 99/00549 to Deka, et al. Processes in which polymer is applied to a formed web are generally referred to herein as “latex saturation” processes. The term “latex” as used herein refers to a synthetic polymer emulsion. Processes in which polymer is applied to the fibers before the web is formed are generally referred to herein as “wet end deposition,” the term “wet end” referring to the section of the paper machine.
Examples of latex-saturated substrates include products designated as BP 336 and BP 321 that are available from Kimberly-Clark Corporation. These products are base papers that may be used as medical packaging substrates and comprise various amounts of cellulosic pulps and synthetic latex.
In addition to being permeable to sterilizing gases and relatively impermeable to bacteria, the fibrous webs should be strong and should exhibit relatively high internal bonding, or delamination and tear resistance. Surgical instruments and trays containing various surgical instruments are often sterilized while wrapped in the medical packaging substrates. After sterilization, the storage containers may then be placed on shelves in a storage facility for later transportation to the operating room. Because such storage and transportation may involve the bumping or rubbing of the storage containers against one another, the medical packaging substrates must be strong enough to withstand such handling.
In addition, fibrous webs may also possess a certain degree of fluid repellency to prevent further transmission of the bacteria. It is often desired that medical packaging substrate be non-toxic, odorless, lint-free, drapable, supple, smooth, etc. The need for such “touch and feel” characteristics depends on the particular project for which the bacteria barrier fabric is to be used.
Fibrous web packaging substrates may be formed from either cellulosic fibers alone, synthetic polymeric fibers alone, or a combination of both cellulosic and synthetic fibers. For example, U.S. Pat. No. 5,204,165 to Schortmann discloses a nonwoven laminate having barrier properties that is described as being suitable for industrial, hospital, and other protective or covering uses. The laminate consists of at least one thermoplastic fiber layer bonded with a wet-laid fabric layer made from a uniform distribution of cellulose fibers, polymeric fibers, and a binder. In one embodiment, spunbound polyester fiber layers are ultrasonically bonded on each side of a wet-laid barrier fabric made of eucalyptus fibers and polyester fibers. The barrier fabric is bonded with an acrylic latex binder. The binder is added to the formed polymeric/cellulosic web after the web is formed. The binder may be added by any one of several methods, including foamed emulsion, gravure roll polymer emulsion, spraying, padding and nip-pressure binder pick-up. Schortmann is an example of a barrier fabric formed using a latex saturation process.
Another process for saturating a formed web with a latex binder is disclosed in U.S. Pat. No. 5,595,828 to Weber. A polymer-reinforced paper, which includes eucalyptus fibers, is disclosed. After forming the web from eucalyptus fibers and, optionally, other fibers such as non-eucalyptus cellulosic fibers and/or synthetic fibers, the web is saturated with a latex binder.
Various latex emulsions have been used as binder materials for paper-based webs as well as coating materials for nonwoven webs. Polymeric emulsions of acrylates, polymethacrylates, poly(acrylic acid), poly(methacrylic acid), and copolymers of the various acrylate and methacrylate esters and the free acids; styrene-butadiene copolymers; ethylene-vinyl acetate copolymers; nitrile rubbers or acrylonitrile-butadiene copolymers; poly(vinyl chloride); poly(vinyl acetate); ethylene-acrylate copolymers; vinyl acetate-acrylate copolymers; neoprene rubbers or trans-1,4-polychloroprenes; cis-1,4-polyisoprenes; butadiene rubbers or cis- and trans-1,4-polybutadienes; and ethylene-propylene copolymers have been used to saturate paper-based webs in order to enhance strength and delamination resistance.
Latexes have also been used as barrier coatings to form fluid impervious webs. For example, in U.S. Pat. Nos. 5,370,132 and 5,441,056 to Weber et al. a nonwoven material's surface is first treated with a repellent coating material such as fluorocarbon. The treated surface is then coated with a barrier coating which may be one of the various latex emulsions. Unlike a saturated web which will have latex particles throughout the web, the described webs in the Weber et al. patent have a surface barrier coating comprising a latex or other barrier material.
Although many latex-saturated webs perform well enough to function as medical packaging barrier substrates, saturating a cellulose paper web with a polymer emulsion to obtain the necessary strength typically results in reduced barrier efficacy. It is possible to improve barrier by refining the pulp as part of the papermaking process. Refining can be described as a grinding action that separates the pulp into individual fibers and works to free the outer fibrils from the surface of the fiber. This action creates more sites on the fiber for bonding with other fibers and thereby increases the tensile strength and delamination resistance of the web. Refining also reduces the size of open passages through the sheet and thus decreases the porosity or permeability of the sheet. Refinement techniques are well documented in the art and the relationship between parameters of refinement processes and the desired characteristics of resulting webs is well known to persons skilled in the art. One disadvantage of using highly refined webs, however, is that refining tends to reduce the tear resistance of a web. Despite the availability of several alternative bacteria barrier fabrics, a need still exists for further improved medical substrates that can be used in forming bacteria barrier packages.
A disadvantage of using polymer saturated paper as the breathable cover is the absence in the art of a saturant that will confer upon the paper the ability to form a strong adhesive bond with the base components through heat sealing without compromising drapability of the paper. Heat sealing refers broadly to any process involving the creation of an adhesive seal between two objects through the application of heat, often with pressure. In many applications the base component and breathable cover are attached by heat sealing. Some base components are comprised of polymers capable of forming bonds with other polymeric materials by means of heat sealing. Other base components are extruded or coated with an outer layer comprising heat sealable polymers. Examples of polymeric materials found in base components that have such strong sealibility include, but are not limited to polypropylene, polyethylene, (including but not limited to low density, linear low density), and ultra low density polyethylene), various copolymers of vinyl acetate (including, but not limited to, low and high vinyl acetate compositions of ethylene vinyl acetate) and ethylene acrylic acid. Because many polyolefin webs contain polymeric material that forms strong heat seals with materials used in base components, such webs can be heat sealed to base components, often eliminating the need for applying an adhesive coating to the surface of the breathable cover. By contrast, many polymers used to saturate papers for use as breathable covers lack sufficient affinity for heat sealing to materials used in base components and thus cannot form as strong of a bond without the use of a temperature sensitive adhesive coating.
A saturated paper that can form a sufficiently strong bond to the base component through heat sealing could in many cases eliminate the need to coat the saturated paper altogether. Allowing the saturated paper to bond directly to the base component will results in a seal that involves only one interface of different materials rather than two interfaces on either side of the sealant. Eliminating one of the interfaces reduces the potential for seal failure. In addition, removing the coating step would reduces the potential for departures from product specifications due to errors in that step of the process. Examples of production errors associated with coating include “skip coating,” in which the coating is not applied to an entire surface, or the formation of pinholes in the coating. Eliminating the use of the coating and the coating production step would also result in cost savings.
There have been some efforts to develop papers that can be sealed to base components containing heat sealable polymeric materials by impregnating papers with heat sealable polymeric materials. For example, International Publication Number WO 98/24970 to Cohen et al. teaches impregnating papers with a polymer emulsion primarily for the purpose of improving strength. Cohen et al. discusses heat sealability of the impregnated papers and includes examples that involve impregnating paper with ethylene acrylic acid and polyethylene, two heat sealable saturant materials.
In practice, however, saturating paper with heat sealable polymeric materials has resulted in webs that have less than desirable drapability for packages containing some medical products. “Drapability” refers essentially to flexibility and absence of stiffness in a fibrous web. Sufficient drapability allows a web to conform to the contours of the products contained in the package and thus to assure a higher degree of contact between the web and the surface area of the product. A softer, more drapable web is less brittle and more flexible and would therefore provide for easier handling of flexible packages with less potential for puncture or tear. Coatings applied to saturated papers in the past have served not only to promote heat sealing but also to enhance the paper's function as a barrier to pathogen contamination. Even for a saturant that forms stronger bonds when heat sealed, there may be a need for a temperature sensitive adhesive coating that is compatible with the saturant for use in some applications in which it is desirable to increase the seal strength further, to improve barrier properties, or both. What is needed in the art therefore is a saturant that may be used to confer heat seability on a paper without compromising the drapability of saturated paper. A paper saturated with such a saturant could be sealed without the need for an adhesive coating and would have utility for medical products and other products for which drapable heat sealable packaging is desired. In the event that a need to use the paper along with a coating is found to exist, there will be a further need in the art for a coating that is compatible with the standard paper.
What is further needed in the art are substrates that readily allow sterilization materials to enter into the package and sterilize the enclosed appliances while at the same time exhibiting sufficient strength, at least in terms of delamination and tear resistance, to function as medical packaging. In particular, a need exists for maintaining the barrier efficacy of latex-saturated webs without hindering the enchanced strength of these webs resulting from latex-saturation without additional refinement. Any webs that allow for sufficient amounts of latex add-on without decreasing barrier efficacy would be improvements over known latex-saturated substrates used as medical packaging.